In the xerographic method of electrophotography the free surface of a photoconductive material, such a amorphous selenium, As.sub.2 Se.sub.3, ZnO or a sensitized polymer, which is supported by a substrate that conducts electricity rather well, is corona-charged to a certain surface potential. Subsequently the charged surface is exposed to a light and dark image formed by reflection of light from a document to be copied. On the dark places the charge remains, whereas on the illuminated places a reduction of the surface potential takes place due to a photocurrent normal to the surface. The resulting distribution of the surface potential replicates the light and dark places of the document. The electrostatic image formed is then developed by e.g. applying a toner with an opposite charge on the surface with the electrostatic image. On the charged places the toner adheres, whereas on the other places it does not adhere, so that a toner image is formed which may be reproduced e.g. on paper.
From the above it is clear that a satisfactory electrophotographic coating must accept charge readily, must retain its charge on the non-illuminated phase (i.e. it must have a low dark decay), must loose its charge rapidly and as completely as possible on the illuminated places (i.e. it must have excellent light discharge characteristics) and must give a uniform photoresponse in the entire visible region and on all parts of the coating. Moreover, the coating must be suitable for frequent use, must adhere well to the substrate and must be resistant to abrasion and scratching.
As a substrate among others metals, such as aluminium, and paper are used.
Numerous literature places describe photoconductive materials. More recently polyimides have been described for this purpose (U.S. Pat. No. 3,554,744, Japanese patent publication No. 24.754/68, Japanese patent application Nos. 73.43145 and 74.11591 and Research Disclosure, nr. 105, January 1973, article 10503). However, it appeared in practice that these polyimides are not satisfactory, because their photosensitivity is insufficient, especially in the visible region.
A more promising development has been described in the U.S. Pat. No. 3,484,237 and an article of R. M. Schaffert in IBM J. Res. Develop., January 1971, pages 75-89, which publications disclose a photoconductor consisting of poly-N-vinylcarbazole containing 2,4,7-trinitro-9-fluorenone (TNF). Preferably one molecule of the TNF is used for one monomer unit of N-vinyl-carbazole. These publications describe that for said compositions having an 1:1 molar ratio the photosensitivity is greater and the dark decay is slower for negative corona charging than for positive corona charging. However, with a decreasing TNF content, the positive charge acceptance increases and the negative charge acceptance decreases. The cross over point occurs at a TNF concentration of about 0.06 (mole of TNF per monomer unit of N-vinylcarbazole), which corresponds to about 10% by weight of TNF, based on the poly-N-vinylcarbazole.
The U.S. Pat. Nos. 3,408,185 and 3,408,189, respectively, describe the use of Lewis acids, among which 2,4,7-trinitro-9-fluorenone is one of the preferred compounds, as photosensitizers in polyurethane resins and melamine resins respectively. They also show that the addition of a Lewis acid to an inert resin, such as an ethylmethacrylate resin, does not result in photosensitive response.
British Pat. No. 1,150,435 describes a process for preparing a photoconductive material by contacting an organic polymeric resinous film, which is capable of retaining an electrostatic charge in the absence of actinic radiation, with a solution containing an impregnation agent which imparts photoconductivity to the film, the solvent in said solution being substantially inert with respect to said film, but being capable of dissolving the impregnation agent. Thereby the impregnation agent is dispersed into at least a portion of the film and the impregnation is continued until the desired degree of photoconductivity is imparted to said film. In one of the examples a solution of 2,4,7-trinitrofluorenone in benzene is refluxed in contact with a polyimide film upon an aluminium substrate. The impregnated film is dried, charged to 1000 Volts by means of a corona discharge device and then exposed to a light and shadow pattern by means of a high pressure mercury vapor lamp. However, when repeating this experiment it has been found that hardly any TNF has been incorporated into the polyimide film and that the resulting film did not show any photosensitivity upon exposure to a light source in the visible region.